Hi, neighbor. A few years ago I tried putting in 2x4’s flat side up, 2x4’s narrow side up, and tree branches. Chickens are creatures of habit so I moved them around to try to eliminate that effect. What I’ve determined is that my chickens don’t care. But some people care passionately about one or the other. My suggestion is to use whatever you feel comfortable with. In Conway you won’t get cold enough to have to worry about them covering their feet. Besides, when mine squat on the tree branches their feet are covered anyway.
This shows how I made mine removable yet locked in place. I drilled a hole through the roost and support and used a large nail, though you could use a large bolt, maybe with a washer. You can get hangers at a hardware store to support a 2x4 on edge. For a 2x4 flat maybe notch out a hole the 2x4 fits in (1-1/2” x 3-1/2”) or use 3 different pieces of wood to create that size gap. It’s hard to explain but just make something that gravity will hold it in place. There are plenty of different ways to do this.
The way you keep them from wobbling is to use a piece strong enough for the span. A 2x4 on end will be stiffer than a 2x4 flat, but a 2x4 flat will still span a reasonable distance without much deflection.
Most chickens like to roost on the highest place available, all other things being equal. But occasionally you get some that don’t, for whatever reason. Maybe it is a Silkie that can’t fly. Maybe there is a bully that beats a chicken up if it tries to roost up high. Maybe they just don’t want to. They don’t always explain why just like they don’t listen when you try to explain things to them.
I set my heights in a coop by determining the height of the floor, including any bedding. My pop door and other doors are high enough that the bedding doesn’t get scratched out. I determine the height of my nests. Some people are happy with nests on the floor, some like them high enough so they don’t have to bend over to gather eggs. Do you maybe have a bad back? In my opinion, nest height is something else people tend to worry about more than the chickens. Then make the roosts higher than the nests. That doesn’t always work but it goes a long way in keeping them from sleeping and pooping in your nests.
I like the roosts as low as I reasonably can and still get them higher than the nests. If you feed your chickens so they are really large for the breed, it’s possible they can injure their legs getting off the roosts. Mine aren’t that big but the reason I like the roosts lower is that the higher the roost, the more clear space you need for them to land. They are going to spread their wings and fly down. They also spread their wings when they fly up. They need clear space so they can spread their wings without banging into nests, feeders, waterers, or walls. My roosts are five feet off the coop floor. My chickens are full sized, not bantams.
Roosts should be at least a foot away from a wall. If you use two roosts, separate them by at least a foot. A little more won’t hurt but 12” should be enough.
One option, depending on the coop, is to put your roosts over that shelf and use the shelf as a droppings board if it is wide enough. Or maybe use the shelf as a support for that side of your tray.